A father's death isn't necessary for a boy to become a man. Most of us don't lose our fathers until we're middle aged. Are we boys until then?

In real life, no.

In fiction, yes. That's why there are so many orphans in comics.

Writers have to be cruel with their characters because a good script works like an emotional rollercoaster.

It is even truer in movies than comics.

Just like soap opera, in comics, the writers prefer to work with characters on the long run. On the contrary, in movies (especially mainstream blockbusters), the writer needs at one time or another a huge "firework" (the climax) and the death of one of the main characters often fits the bill.

Interestingly enough, there is even a real firework after the death of Yondu in the movie. It may be some kind of mise-en-abime or the blinking of an eye by the director.